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        <td class="header">SAR Simulation</td>
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<h3>SAR Simulation Operator</h3>

<p>
    The operator generates simulated SAR image using DEM, the Geocoding and orbit state vectors
    from a given SAR image, and mathematical modeling of SAR imaging geometry. The simulated SAR
    image will have the same dimension and resolution as the original SAR image.</p><h4>Major Processing Steps</h4>

<p>Some major steps of the simulation procedure are listed below:</p>
<ol>
    <li>First&nbsp;a DEM image is created from the original SAR image. The DEM image
        has the same dimension as the original SAR image. The pixel value of the DEM image is the
        elevation of the corresponding pixel in the original SAR image.<a href="SARSimulationOp.html"></a></li>
    <li>Then, for each cell in the
        DEM image, its pixel position (row/column indices) in the simulated SAR image is computed
        based on the SAR model.
    </li>
    <li>Finally, the backscattered power &#963;<sub>0</sub> for the pixel is computed&nbsp;using backscattering model.
    </li>
</ol>
<h4>DEM Supported</h4>

<p>Right now only the DEMs with geographic coordinates <span style="font-size: 11pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">(</span><i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">P</span></i><span style="font-size: 7pt; font-family: Symbol;" lang="EN-GB">&#1488;</span><i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">, P</span></i><span style="font-size: 7pt; font-family: Symbol;" lang="EN-GB">&#1488;</span><i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">, P</span><span style="font-size: 8pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">h</span></i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">)</span>&nbsp;referred to global
    geodetic ellipsoid reference WGS84 in meters are properly supported.&nbsp;</p><p>Since
    the&nbsp;height information in ACE and SRTM is referred to geoid EGM96, not&nbsp;WGS84
    ellipsoid,&nbsp;correction has been applied to obtain height
    relative to the WGS84 ellipsoid.&nbsp;</p>

<p>User can also use external DEM file which, as specified above, must be WGS84 <span style="font-size: 11pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">(</span><i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">P</span></i><span style="font-size: 7pt; font-family: Symbol;" lang="EN-GB">&#1488; </span><i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">, P</span></i><span style="font-size: 7pt; font-family: Symbol;" lang="EN-GB">&#1488;</span><i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">, P</span><span style="font-size: 8pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">h</span></i><span style="font-size: 12pt; font-family: &quot;Times New Roman&quot;;" lang="EN-GB">)</span>&nbsp;DEM in meters.</p>
<h4>Layover-Shadow Mask Generation</h4>

<p>Besides producing simulated image, this operator
    can also generate layover-shadow mask for the simulated image using the
    2-pass algorithm given in section 7.4 in [2]. For details of
    layover-shadow mask generation, reader is referred to <a href="SARSimTerrainCorrectionOp.html">SAR Simulation
        Terrain Correction operator.</a></p><h4>Parameters Used&nbsp;</h4>

<p>The following parameters are used by the operator:
</p>
<ol>
    <li> Source Band: All bands (real or virtual) of the source product. User can select one
        or more bands for producing simulated image. If no bands are selected, then by default all
        bands are selected. The selected band will be output as a band in the target product
        together with the simulated image.
    </li>
    <li> Digital Elevation Model: DEM types. Choose from the automatically
        tiled DEMs or specify using a single external DEM file by selecting
        "External DEM".
    </li>
    <li> DEM Resampling Method: Interpolation method for obtaining
        elevation values from the
        original DEM file. The following interpolation methods are available:
        nearest neighbour, bilinear, cubic convolution, binsinc and bicubic
        interpolations.
    </li>
    <li> External DEM: User specified external DEM file. Currently only WGS84-latlong
        DEM&nbsp;in meters is accepted as geographic system.
    </li>
    <li>Save Layover-Shadow Mask as band: Checkbox indicating that layover-shadow mask is saved as a band in the target
        product.
    </li>
</ol>
<br>

<p><img style="width: 500px; height: 501px;" alt="" src="images/sar_simulation_dlg.jpg"></p>

<p></p><h4>Detailed Simulation Algorithm</h4>

<p>Detailed procedure is as the follows:</p>
<ol>
    <li>Get data for the following parameters from the metadata of the SAR image product:</li>
    <ul>
        <li>radar wave length</li>
        <li>range spacing</li>
        <li>first_line_time</li>
        <li>line_time_interval</li>
        <li>slant range to 1st pixel</li>
        <li>orbit state vectors</li>
        <li>slant range to ground range conversion coefficients</li>
    </ul>
    <li> Compute satellite position and velocity for each azimuth time by interpolating the
        orbit state vectors;
    </li>
    <li> Repeat the following steps for each cell in the DEM image:</li>
    <ol type="1)">
        <li>Get latitude, longitude and elevation for the cell;</li>
        <li>Convert (latitude, longitude, elevation) to Cartesian coordinate P(X, Y, Z);</li>
        <li>Compute zero Doppler time t for point P(x, y, z) using Doppler frequency function;</li>
        <li>Compute SAR sensor position S(X, Y, Z) at time t;</li>
        <li>Compute slant range r = |S - P|;</li>
        <li>Compute bias-corrected zero Doppler time tc = t + r*2/c, where c is the light speed;</li>
        <li>Update satellite position S(tc) and slant range r(tc) = |S(tc) &#8211; P| for the bias-corrected zero
            Doppler time tc;
        </li>
        <li>Compute azimuth index Ia in the source image using zero Doppler time tc;</li>
        <li>Compute range index Ir in the source image using slant range r(tc);</li>
        <li>Compute local incidence angle;</li>
        <li>Compute backscattered power and save it as value for pixel ((int)Ia, (int)Ir);</li>
    </ol>
</ol>
<p><span style="font-style: italic;">Reference:</span></p>

<p>[1] Liu H., Zhao Z., Lezek K. C., Correction of Positional Errors and Geometric Distortions
    in Topographic Maps and DEMs Using a Rigorous SAR Simulation Technique, Photogrammetric
    Engineering &amp; Remote Sensing, Vol. 70, No. 9, Sep. 2004</p>

<p>[2] Gunter Schreier, <span style="font-style: italic;">SAR geocoding: data and systems</span>, Wichmann-Verlag,
    Karlsruhe, Germany, 1993</p>
<br>

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